<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <meta http-equiv="Content-Style-Type" content="text/css"> <link rel="up" title="FatFs" href="../00index_e.html"> <link rel="alternate" hreflang="ja" title="Japanese" href="../ja/dread.html"> <link rel="stylesheet" href="../css_e.css" type="text/css" media="screen" title="ELM Default"> <title>FatFs - disk_read</title> </head> <body> <div class="para func"> <h2>disk_read</h2> <p>The disk_read function is called to read data from the sector(s) of storage device.</p> <pre> DRESULT disk_read ( BYTE <span class="arg">pdrv</span>, <span class="c">/* [IN] Physical drive number */</span> BYTE* <span class="arg">buff</span>, <span class="c">/* [OUT] Pointer to the read data buffer */</span> DWORD <span class="arg">sector</span>, <span class="c">/* [IN] Start sector number */</span> UINT <span class="arg">count</span> <span class="c">/* [IN] Number of sectros to read */</span> ); </pre> </div> <div class="para arg"> <h4>Parameters</h4> <dl class="par"> <dt>pdrv</dt> <dd>Physical drive number to identify the target device.</dd> <dt>buff</dt> <dd>Pointer to the first item of the <em>byte array</em> to store read data.</dd> <dt>sector</dt> <dd>Start sector number in 32-bit LBA.</dd> <dt>count</dt> <dd>Number of sectors to read.</dd> </dl> </div> <div class="para ret"> <h4>Return Value</h4> <dl class="ret"> <dt>RES_OK (0)</dt> <dd>The function succeeded.</dd> <dt>RES_ERROR</dt> <dd>A hard error occured during the read operation and could not recover it.</dd> <dt>RES_PARERR</dt> <dd>Invalid parameter.</dd> <dt>RES_NOTRDY</dt> <dd>The device has not been initialized.</dd> </dl> </div> <div class="para desc"> <h4>Description</h4> <p>The data read/write operation to the storage devices is done in unit of <em>sector</em>. FatFs supports the sector size in range of from 512 to 4096 bytes. When FatFs is configured to fixed sector size (<tt>_MIN_SS == MAX_SS</tt>, this will be the most case), the read/write function must work at that sector size. When FatFs is configured to variable sector size (<tt>_MIN_SS != MAX_SS</tt>), sector size is inquired with <tt>disk_ioctl</tt> function following <tt>disk_initialize</tt> function.</p> <p>The memory address specified by <tt class="arg">buff</tt> is not that always aligned to word boundary because the argument is defined as <tt>BYTE*</tt>. The unaligned read/write request can occure at <a href="appnote.html#fs1">direct transfer</a>. If the bus architecture, especially DMA controller, does not allow unaligned memory access, it should be solved in this function. There are some workarounds described below to avoid this issue.</p> <ul> <li>Convert word transfer to byte transfer in this function if needed. - Recommended.</li> <li>On the <tt>f_read()</tt> calls, avoid long read request that includes a whole of sector. - Any direct transfer never occures.</li> <li>On the <tt>f_read(fp, data, btw, bw)</tt> calls, make sure that <tt>(((UINT)data & 3) == (f_tell(fp) & 3))</tt> is true. - Word alignment of <tt class="arg">buff</tt> is guaranteed.</li> </ul> <p>Generally, a multiple sector transfer request must not be split into single sector transactions to the storage device, or you will not get good read throughput.</p> </div> <p class="foot"><a href="../00index_e.html">Return</a></p> </body> </html>